Biodiesel, as renewable fuel, has been used in conventional diesel engines in the pure form or as biodiesel/diesel blends for many years. However, thermal stability of biodiesel has been minimally explored. In this study, thermal stability of soybean oil based commercial biodiesel was investigated. Thermal stressing of biodiesel was performed in batch reactors at 250 - 425 °C for a residence time varying from 3 to 63 min. Biodiesel samples were collected and analyzed by GC-FID and GC-MS. It was observed that biodiesel remained stable at 275 °C, and the stability reduced as temperature and residence time increased. The GC-FID and GC-MS analyses showed that biodiesel degradation at high temperatures involves mainly three types of reactions: isomerization reactions to form C18:2 isomers, the Diels-Alder reaction between C18:1 and C18:2 to form dimers, and thermal decomposition reactions to form smaller FAMEs, hydrocarbons and gas products. Accordingly, a six-lump model was proposed to describe the mechanism of biodiesel degradation. This model was further simplified to a three-lump model and then to the general first order reaction model. Finally, kinetics of biodiesel decomposition was simulated using both reversible and irreversible first order reaction models, and results show that the reversible model was superior to the irreversible one.